1. Field of the Invention
The present invention relates to a semiconductor device having a Heterojunction Bipolar Transistor (HBT) structure and, more particularly, to a transistor structure and manufacturing method thereof, in which compressive or tensile stress is additionally induced from outside to modify physical and electric properties of a semiconductor layer, thereby improving the performance of the transistor.
2. Description of the Related Art
The possibility of a SiGe HBT was proposed in 1957, but owing to insufficient understanding to physical properties of semiconductor and poor technologies about HBT epitaxial growth and manufacturing process, a long time has been spent to develop HBTs and enable their use. Accordingly, SiGe low temperature growth applicable to a transistor was published in 1981, and operating SiGe HBT was published in 1987 by Meyerson of IBM, who developed Ultra High Vacuum Chemical Vapor Deposition (UHVCVD). In 1990, several researchers achieved ft to exceed 75 GHz. In 1995, Daimler-Chrysler published high frequency characteristics of ft/fmax=130/160 GHz, which present applicability to millimeter waves. During these processes, various technologies for SiGe epitaxial growth have been studied, which include UHVCVD, Molecular Beam Epitaxy (MBE), Reduced Pressure Chemical Vapor Deposition (RPCVD), Low Pressure Chemical Vapor Deposition (LPCVD) and the like. Thanks to these researches, it is started to develop SiGe Bipolar Complementary Metal-Oxide Semiconductor (BiCMOS) in 1992. In 2004, very high ft of 375 GHz was published. In practice, SiGe ICs have been fully commercialized since 1998 as Low Noise Amplifier (LNA), mixer, power amplifier, Voltage Controller Oscillator (VCO) and the like were launched by IBM, Temic, Maxim, SGS Thomson and so on.
However, there are many rooms to improve epitaxy technologies and transistor structures. That is, it is required to decrease current leak and raise reliability in VLSICs. In particular, advanced technologies commonly usable with CMOS process technologies are required to be developed to reduce process steps. In addition, as entering nano-scale as in CMOS technologies, research and development for process and design are consecutively required to produce transistors having a more minute structure.
HBT having merits of a heterojunction structure generally enhances the mobility of carriers to improve high speed operation characteristics and reduces non-linear components caused by single channel to improve characteristics. A conventional SiGe HBT structure has some problems that have to be improved such as large leakage current between a base and a collector and large capacitance of junction, which restricts maximum allowable frequency. These problems act as obstacles against the commercialization of SiGe-HBT, which has been researched by a number of researchers for many years.
Conventionally, in order to raise the speed of a bipolar transistor, a base and a collector have been reduced in their thickness. In addition, to enhance gains, the concentration of impurities implanted into the emitter and the base was controlled or an emitter-base junction of a heterojunction structure has been generally used. However, the thickness of the base cannot be reduced limitlessly and the adjustment of doping concentration or the controlling of a heterojunction structure is also restricted owing to material properties.
FIGS. 1a to 1c are cross-sectional views illustrating conventional transistors, in which FIG. 1a shows a bipolar transistor, FIG. 1b shows a simple structure of HBT, and FIG. 1c shows a transistor with a trench used between a substrate electrode and a metal line of the transistor to decrease capacitance. Here, bases 111 and 112 are formed on a substrate 113 via ion implantation, by using an insulating layer 113.
A Bipolar Junction Transistor (BJT) shown in FIG. 1a, based on only a common bulk substrate, is generally used for low speed analog circuits. At transistor ft of about 50 GHz, the BJT is being replaced by a Heterojunction Bipolar Transistor (HBT) shown in FIG. 1b. This is primarily because the HBT has an epitaxial base 122 replacing a base layer of the BJT obtained by ion implantation. In the HBT, high gain is obtained by a large offset of a balance band in an interface of base-emitter 122, which obstructs positive hole implantation, and an offset in a conduction band is small to reduce turn-on voltage, thereby decreasing power consumption. In addition, the base having a thickness of 5 to 10 nm, doped with a high concentration, decreases the alteration of the base to enhance linearity characteristics and raise ft, but maximize fmax by reducing base resistance.
In the conventional bipolar transistors, performance has been enhanced by reducing the transistor size or by adopting the HBT that uses a heterojunction structure.
However, the operation is still determined by the physical properties of materials constituting the base and the collector, and thus there are needs for another method and transistor structure change that can further enhance performance.
Of the cross reference to the related art, [1] (hereinafter only the document number will be described) discloses a SiGe HBT structure of IBM, in which isolation is enabled by Shallow Trench Isolation (STI) and Deep Trench Isolation (DTI). A triangular SiGe inner base is epitaxially grown, an oxide layer is formed by high pressure oxidation (HIPOX), and then an outer base is deposited. Then, the outer base and the oxide layer are etched in regions around the emitter, a nitride film sidewall is formed, and then an emitter is formed by self-alignment. This transistor structure has merits of simple heat treatment, low stress level and correct control of an emitter cap.
[2] discloses a SiGe HBT structure of Matzushida, in which STI and DTI are applied, an inner base is grown by Ultra High Vacuum Selective Epitaxial Growth (UHV SEG), an outer base is formed, and after etching, an emitter is produced through self-alignment by using an insulating sidewall. The SiGe base is composed of SiGe-cap, graded-SiGe and SiGe-spacer layers, in which the graded-SiGe layer is formed of a number of very thin SiGe layers, which are layered repeatedly. The base structure like this is optimized to increase Fmax.
[3] discloses a SiGe HBT structure of Hitachi, in which a p-SiGeC/n-SiGe structure is applied to an emitter-base junction to enhance electron implantation efficiency in an emitter. An outer base is etched, an inner base is formed by selective growth using an oxide layer sidewall, and the emitter is produced by self-alignment. The SiGe layer of the base is represented in a quadrangular and diamond structure, and the content and topology of C in the front side is optimized to enhance electron implantation efficiency.
[4] is a patent of IBM, proposing a transistor structure, which can enhance transistor performance by controlling the mobility of electrons based on the stress of an outer base in manufacturing of a SiGe HBT. This is difficult to manufacture because of a complicated transistor structure, and an outer base for inducing stress is grown across a wafer. Since thickness to be used is restricted, it is difficult to efficiently control stress.
[5] discloses a representative SiGe HBT structure of IBM, in which isolation is made by STI and DTI, and a triangular SiGe base is used. Low noise characteristics are improved by using a poly-Si emitter of 0.5×20×62, doped with As.
[6] discloses a SiGe HBT structure of Hitachi, in which trench isolation is performed, an inner insulating layer sidewall is used to produce an emitter, an inner base is selectively grown to join with the outer base, and then the emitter is self-aligned. Base-collector capacitance was set 1 fF for the purpose of ultra low voltage and high speed operation. This structure operates with a frequency of 30 to 70 GHz at 100.
[7] discloses a representative transistor structure of Daimler-Benz. A SiGe HBT is developed by MBE for high frequency operation of a Si bipolar transistor, which can operate with a frequency up to 100 GHz. A base, an emitter and a collector are discriminatively produced by mesa etching, in which the base structure is controlled to be quadrangular, inner surface resistance of the base is controlled to be 0.7-3 kohm/sqr.
[8] is a report from IBM, representing results about characteristics change in a case where SiGe BiCMOS is manufactured using a strained epitaxial layer, which is used in MOS. HBT performance degradation caused by tensile stress used in FET is analyzed.
Compared to the conventional technologies, the present invention proposes a high performance HBT, and more particularly, an advanced transistor structure and a manufacturing method thereof, in which compressive or tensile stress is additionally induced from outside to alter physical and electric properties of a semiconductor layer, thereby enhancing transistor performance. Accordingly, the present invention differs from the conventional approaches [1] to [8] in that stress is locally and additionally used to enhance transistor performance, energy band difference is controlled, and different processes are used to manufacture this transistor structure.